JP 2009-42822A discloses a power line communication apparatus. According to this conventional power line communication apparatus, certification information is communicated between a power supply device and a load device to certify that the power supply device and the load device are in a predetermined normal combination. It is proposed to provide a switch in a plug receptacle and a power plug to output a signal when the power plug is plugged into the plug receptacle and perform power line communication in response to this signal.
According to the conventional device, a special switch need be formed in the plug receptacle and the power plug. It is therefore not possible to use a general-purpose plug or plural types of plugs. The special switch increases costs of a system. It is likely that the switch in a mechanical construction is likely to operate erroneously because of foreign matters for example. It is therefore difficult to realize high reliability.
Although it is possible to detect a connection of a power line by using a communication function of a power line communication apparatus, it is necessary to supply electric power to the power line communication apparatus even when the power line is not connected. This will result in increase of power consumption.